In recent years, the oil-gas insulation system has been replaced by optical isolation technologies in high voltage and extra-high voltage electronic current transformers, so that the insulation technology of new type transformer has achieved a substantial development. The key for implementing such technology development is that, there are optical fiber communications between the high voltage side and low voltage side, and a power supply for a digital circuit is required to be set at the high voltage side, so as to digitize the magnitude of current; this is the biggest technical problem confronted during the manufacturing of the electronic transformer. At present, the method of transporting laser energy from the low voltage side to the high voltage side via the optic cable is generally adopted by domestic and foreign researchers and developers. The disadvantages for such method include: {circle around (1)} the life period of the laser source is short with about only 8000-10000 hours; {circle around (2)} the cost is expensive at about 12,000¥, which is more than half of the cost of a whole 110 kv transformer; {circle around (3)} the environmental condition required by the laser is strict, as a result, it is difficult to conduct the on-site operation and maintenance. Therefore, researchers hope to obtain energy serving as the high voltage side power supply directly from the magnetic field of high voltage bus bar which is to be measured, via electromagnetic induction. There are already some domestic patents and technical literatures in the field of energy supply depending on the self-excitation power supply, for example, a paper titled “research for the energy supplying method of self-excitation power supply at the high voltage side of the electronic current transformer” was published in High-voltage Electrical, 2006, Issue 1, and a patent titled “a self-excitation power supply apparatus obtaining energy from the power line magnetic field (ZL2005100430602)”. At that time, the wake-up current is reduced to less than 800 mA with the use of the technical measure, but the problem of wake-up time for the transformer has not resolved completely yet. The startup time is postponed for more than 50 mS since the output voltage of the energy-obtaining coil is required to be rectified and filtered after each switching on. To overcome this shortcoming and reduce the startup time, an external laser power supply is still required at the startup time (i.e. the time of each switching on) of the transformer, which is a big disadvantage and hampers the usage of transformer.